Without limiting the scope of the present invention, its background will be described in relation to systems and methods for post combustion mercury control using sorbent injection and wet scrubbing, as an example.
With the introduction of the first national standards for mercury pollution from power plants in December of 2011, many facilities will turn to sorbent injection to meet the EPA Mercury and Air Toxics Standards (MATS) requirements. Sorbent injection is a technology that has shown good potential for achieving mercury removal to the MATS standards.
While several sorbents are viable for sorbent injection, activated carbon (AC) has been proven to the largest extent. Activated carbon is a high surface area sorbent typically created from the activation of coal (or other material high in carbon content) in a controlled environment to create a porous network. This porous network and chemical activity of the AC can be manipulated during activation/manufacturing to create an AC that will preferentially adsorb certain contaminants of concern (e.g., mercury from power plant flue gas to meets MATS standards). Additionally, post activation treatment can be performed to enhance the chemical reactivity of the AC for the target compound(s) of interest. For sorbent injection, the AC is ground and sized to produce powdered activated carbon (PAC), most typically to 95% passing the 325 mesh for mercury capture from flue gas.
Many efforts have been made to improve PAC materials to increase the mercury capture potential and thereby decrease the PAC loading to reduce materials handling and cost burdens. For example, U.S. Pat. No. 6,953,494 describes treating a carbonaceous substrate with an effective amount of a bromine-containing gas; U.S. Pat. No. 8,551,431 describes a sorbent with halogens applied with washing; U.S. Pat. No. 8,057,576 describes a dry admixture of activated carbon and halogen-containing additive; and U.S. Pat. No. 8,512,655 describes a carbon promoted by reaction with a halogen or halide and possibly other components to increase the reactivity of the sorbent. Other attempts have been made to improve the mercury removal from power plant flue gas using halogen additives to the power plant process itself. For example, U.S. Pat. No. 8,524,179 describes adding iodine or bromine to the feed material; and U.S. Pat. No. 8,679,430 describes injecting a halogen compound into the combustion chamber and/or exhaust stream.
All of these presented disclosures rely on halogen additives to improve mercury capture. Since bromine is a strong oxidant, it can also cause oxidation and corrosion of the duct system and other equipment with which it comes into contact, causing increased maintenance and cost. Further, there are currently no monitoring requirements for bromine compounds; but if emitted to the atmosphere, it would be detrimental to the environment (e.g., ozone depletion in the air and reaction to form carcinogenic compounds in water). Therefore, it would be advantageous to use alternative methods to reduce sorbent injection rates and still achieve low mercury emissions.
Sorbent injection, as applied for control of mercury for MATS compliance, typically involves the pneumatic conveyance of a powdered sorbent from a storage silo into the process gas of a power plant's flue duct downstream of the boiler and upstream of a particulate control device such as an electrostatic precipitator (ESP) or fabric filter (FF). Once introduced to the process gas, the powdered sorbent disperses and adsorbs mercury and other unwanted constituents in the flue gas. The powdered sorbent with adsorbed mercury (and other constituents) then is captured and removed from the gas by a particulate control device.
In coal-fired power plants, mercury capture sorbents typically will be co-collected with other particle matter such as fly ash in an electrostatic precipitator, fabric filter, an electrostatic precipitator in series with a fabric filter, two electrostatic precipitators in series, two fabric filters in series, or similar devices. At this typical injection location (upstream of a particulate collection device), the sorbents' capacity for mercury is limited by the temperatures naturally present (e.g., greater than 350° F.) as the injected sorbents physically and chemically adsorb mercury through endothermic processes. In such a configuration, the time between the injection point and collection point typically is less than three seconds. Therefore, the adsorption of mercury is limited by diffusion and reaction kinetics possible in this short time. Alternatively, if a fabric filter is used as the particulate control device, longer residence times can be realized. This technique is not preferred due to the high cost to install and operate fabric filters as the primary particulate control device.
A drawback to co-collection of sorbents with fly ash has arisen in some scenarios when fly ash is sold as a commodity product. Comingling the sorbent and fly ash makes the mixture of a quality no longer acceptable to sell. To alleviate this issue, two particulate control devices may be employed in series with the second being a fabric filter and sorbent injection for mercury control between the two. This technique segregates the sorbent from fly ash collection and allows for longer contact times for the sorbent to collect mercury. While effective, the capital expenditure, additional operational costs, and pressure drop of the additional fabric filter unit are exorbitant and increase the cost of control. Similarly, sorbent might be injected into the later sections of an electrostatic precipitator so as to try to segregate fly ash material and sorbent. This method, however, even further limits residence time for the carbon to remove mercury, as compared to traditional injection upstream of the electrostatic precipitator, so often would not improve mercury removal or injection rates necessary to substantially reduce mercury emissions.
After exiting the particulate control device, the process gas continues through flue gas ducts with decreased levels of mercury and other constituents. At this point, it is either emitted out of the stack or perhaps passes through a wet flue gas desulfurization (WFGD) unit when installed. Wet flue gas desulfurization units are currently installed on over 50% of the MW capacity in the United States to reduce sulfur dioxide (SO2) emissions. While intended for SO2 capture, mercury also can be captured in the wet flue gas desulfurization units. A high percentage of mercury in the flue gas will partition to a wet flue gas desulfurization liquid when it is found in the oxidized form, but the elemental mercury will pass through without capture. Once oxidized mercury is captured in the liquid, however, it can be reduced by chemical reactions to elemental mercury and leave the stack, referred to as “mercury re-emission.” Sorbent introduced in the wet flue gas desulfurization liquid could sequester mercury species already present in the liquid stream and minimize re-emission of mercury from wet flue gas desulfurization units. While this is positive, a significant portion of the mercury (the uncaptured oxidized and elemental mercury fractions) in the original flue gas could “bypass” the wet flue gas desulfurization unit and still contribute to stack emissions.
The above-described injection locations in coal-fired power applications can have some disadvantages. First, as the powdered sorbent mixes with the fly ash, it changes the properties of the mixture that can affect the salability of this byproduct. For example, fly ash often is sold for use as a cement additive. During concrete production, an air-entrained admixture (AEA) is also added to develop strength properties. When powdered sorbents are mixed with fly ash, especially PAC, they can adsorb the AEA, diminishing its effectiveness and requiring more AEA to be added. Increases in AEA add to cost and thereby may prohibit the sale of fly ash for a cement additive. For facilities that sell fly ash, a solution other than a typical PAC injection must be applied to preserve these byproduct sales.
Second, for most facilities, sorbent injection is a retrofit technology applied to the existing infrastructure. Injection locations have to be installed within existing duct networks that may have poor mixing or residence time necessary for high mercury removal.
In summary, sorbent injection is a proven effective way to remove mercury; however, for some applications, the amount of PAC required can be very high and, therefore, costly (e.g., because of the high temperatures, short residence times, and numerous other complicating factors).